1. Field of the Invention
The present invention relates to a hot isostatic pressing apparatus (HIP apparatus) for economically treating a large quantity of products which can be conveyed in a heated state and in the atmospheric pressure such as, for example, aluminum and magnesium casting products and which are subjected to a hot isostatic pressing (HIP) at a temperature of not higher than 600° C.
2. Description of the Related Art
Hot isostatic pressing (HIP) has come to be widely used for the removal of shrinkage cavity and gas pores from casting products and for the removal of pores remaining in the interiors of sintered products such as ceramics and powder metallurgy products. However, the treatment cost is high which is attributable to a long cycle time, and therefore the utilization of hot isostatic pressing has heretofore been limited to the production of products having a very high added value.
A long time required for cooling after retention at predetermined temperature and pressure and a long heating time required from a relation to heat capacity in a certain quantity of workpieces have been pointed out as causes of a long cycle time.
For solving this problem there have been proposed a preheating type HIP apparatus and a modular HIP apparatus on the basis of the thinking that the time during which workpieces of one batch occupy an expensive high-pressure vessel is to be shortened and that cooling and preheating of workpieces which can be done outside such a high-pressure vessel are to be performed in the exterior of the high-pressure vessel.
In these conventional apparatuses, heaters divided vertically in plural stages are used and temperature control is conducted independently for each stage, that is, plural temperature sensors and plural power sources for heating are required, thus leading to a very high cost.
Moreover, for shortening the cycle time in HIP it is necessary to shorten the high-pressure vessel occupying time of workpieces and therefore, in such a high-efficiency HIP as one cycle time being inside of one hour, it is important that the time for raising pressure and the time for reducing pressure be shortened.
For shortening the pressure raising time and the pressure reducing time it is necessary to inject and discharge a pressure medium gas at high speed into and from a high-pressure vessel. Actually, however, if a pressure medium gas is fed at high speed, there occurs a temperature rising phenomenon due to so-called damming and the temperature of an upper portion of a treatment chamber formed within the high-pressure vessel rises to an excessive degree, so that there occurs a temperature distribution in which it is difficult to control temperature even if electric power to be fed to heaters is controlled. Including this phenomenon, there arises the problem that it is difficult to control the temperature of workpieces, that is, there occurs a large variation in temperature of workpieces and it is difficult to ensure desired mechanical characteristics in the case of a workpiece made of such a material as requires heat treatment.
On the other hand, if HIP is to be followed by a heat treatment such as water quenching or age-precipitation, it is necessary that the workpieces having been subjected to HIP be held at a predetermined temperature. If a pressure medium gas is discharged at high speed, the pressure medium gas present within the high-temperature vessel undergoes a temperature drop due to an adiabatic expansion phenomenon, and thus there is a problem that it is difficult to perform a heat treatment subsequent to HIP.
Moreover, in a preheating type HIP apparatus, workpieces are conveyed in an exposed state to the air, and in a modular type HIP apparatus, heaters used in the HIP apparatus are constructed of a material poor in oxidation resistance, so that it is impossible to expose workpieces to the air at a high temperature and hence in all of the conventional apparatuses it has been impossible to perform a heat treatment subsequent to HIP.
Further, in the conventional apparatuses, since workpieces are conveyed always together with a lower inner lid of a high-pressure vessel, there also is the problem that the workpiece conveying process becomes complicated.